I have three batteries to my E-M5. One original, two "pirate" that I bought with a charger that can charge also the original battery. When I'm out photographing I use to have one spare battery in my pocket. So far this has been enough. But I never shoot video and very seldom bursts. And I use to set the camera to off between the shooting occasions. If it's very could I may have all batteries with me, keeping the spares as warm as possible. A could empty battery can sometimes recover after being treated with some human warmth I have noticed.